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Rainy Areas in U.S. Show Higher Autism Rates
Waldman's group, however, insists that a real increase in the numbers of autistic children cannot be ruled out.
Richard Lathe, an autism expert from Pieta Research in Edinburgh, Scotland, thinks that Waldman might be on to something.
"Nevertheless, one must be vigilant, because statistical correlations do not necessarily imply causality," Lathe said. "This caveat aside, the authors demonstrate, with better than 99 percent certainty, that the correlation is not by chance."
Lathe thinks the most likely explanation for the association between autism and rain is that rain carries chemicals in the atmosphere to the ground.
"This explanation is plausible," Lathe said. "Emissions from manufacturing industries, power plants, and from domestic waste incineration generally rise to the troposphere to be diluted into the large volume of the atmosphere. Precipitation can dump this load back on the land, to be absorbed by plants and animals in the food chain," he said.
One possibility that needs to be addressed further is that exposure need not be in the child, Lathe said. "There has been a suggestion that maternal exposure to environmental toxins might contribute to autism in children," he said.
These results are not definitive evidence in favor of the hypothesis that autism has an environmental trigger, but the results are consistent with the hypothesis, Lathe said. "For the future, one feels it will be essential to study levels of toxins in soil, crop and food samples from the different counties investigated in the Waldman study. A positive correlation would greatly reinforce the environmental hypothesis," he said.
Dr. Noel S. Weiss, a professor of epidemiology at the University of Washington in Seattle, and author of an accompanying journal editorial, thinks the results of the study need to be taken with a grain of salt.
"This is a course analysis," Weiss said. "There are difficulties, because autism is not that unequivocally defined, and the criteria for diagnosis can vary from place to place and over time. The message is really to other scientists who might examine this relationship."
Weiss isn't convinced that the association between autism and precipitation is real. "It could be, but I don't think so," he said. "But it's probably worth looking into."
For more about autism, visit the U.S. National Institutes of Health.
SOURCES: Michael Waldman, Ph.D., professor, economics, Johnson Graduate School of Management, Cornell University, Ithaca, N.Y.; Noel S. Weiss, M.D., Dr.P.H., professor, epidemiology, University of Washington, Seattle; Richard Lathe, Ph.D., Pieta Research, Edinburgh, U.K., and author,Autism, Brain and Environment; November 2008,Archives of Pediatrics & Adolescent Medicine